In recent years, epitaxial wafers where an epitaxial film is formed on the surface of a silicon wafer are widely used as silicon wafers for use with MOS devices. These epitaxial wafers provide improved yield for gate oxidation films of MOS devices, and have superior characteristics such as the reduction of parasitic capacitance, the prevention of soft errors, improved gettering performance, and improved mechanical strength.
With this epitaxial wafer structure, in the prior art where a batch method is implemented so as to perform epitaxial growth process simultaneously on a plurality of silicon wafers, it has become difficult to maintain compatibility with large diameter silicon wafers and single wafer processing epitaxial growth apparatus have therefore mainly been employed. In recent years, epitaxial growth apparatus for use with large diameter wafers capable of performing epitaxial growth process on wafers of a diameter of 300 mm or more have been developed.
With these single wafer type epitaxial growth apparatus, methods of transferring a wafer into and out of the apparatus, and onto a susceptor, can be classified into two types: a type where a wafer is transferred using a Bernoulli chuck method or elevating method using a transportation jig; and a type where the lower surface of the wafer is supported using pins, so that transfer is achieved by raising the pins. However, in each case, a semiconductor wafer is mounted on a single susceptor arranged horizontally in the apparatus. The wafer is then raised to a high temperature using a heat source such as infrared lamps, etc. located around the wafer. Epitaxial growth is then initiated at the wafer surface by flowing a reactive gas over the surface of the wafer at a high-temperature while rotating the susceptor.
The following is a description, with reference to FIG. 19 to FIG. 23, of a susceptor for epitaxial growth and an epitaxial growth method of the prior art.
FIG. 19 is a cross-sectional view schematically showing an epitaxial growth apparatus of the prior art. FIG. 20 is a plane view schematically showing a susceptor for epitaxial growth of the prior art. FIG. 21 is a further cross-sectional view schematically showing a susceptor for epitaxial growth of the prior art. FIG. 22 is a further cross-sectional view schematically showing a design of a susceptor for epitaxial growth of the prior art. FIG. 23 is a further plane view schematically showing of a susceptor for epitaxial growth of the prior art.
As shown in FIG. 19 to FIG. 22, the epitaxial growth apparatus (hereinafter referred to as “apparatus”) 1 internally contains an epitaxial film forming chamber (hereinafter referred to as “film forming chamber”) 2. This film forming chamber 2 is equipped with an upper dome 3, a lower dome 4, and a dome fitting 5. The upper dome 3 and the lower dome 4 are made from a transparent material such as quartz, etc., with a susceptor 10 and silicon wafer W being heated using a plurality of halogen lamps 6 arranged above and below the apparatus 1.
The susceptor 10 is then rotated as a result of an outer part of the lower surface of the susceptor 10 engaging with a support arm 8 linked to a susceptor rotating shaft 7. A carbon base material, coated on the surface with a SiC film, is adopted as the susceptor 10. The susceptor 10 is disc-shaped as shown in FIG. 20, or is disc-shaped having a recess as shown in FIG. 21, and supports the entire rear surface of the silicon wafer W. This recess is comprised of a pocket 10a housing the silicon wafer W and is comprised of a substantially circular bottom wall and a sidewall surrounding this bottom wall. A total of three through-holes 10b are formed every 120 degrees around the outside of the susceptor 10. Elevating pins 9 for raising and lowering the silicon wafer W are inserted loosely at each through-hole 10b. Elevation of the elevating pins 9 is carried out by a lift arm 11.
A gas supply opening 12 and gas exhaust opening 13 are located facing each other at a position of the dome fitting 5 that faces the susceptor 10. Reactive gas, that has been formed by diluting source gas such as SiHCl3, etc. with hydrogen gas (carrier gas) and mixed with a microscopic amount of dopant, is supplied from the gas supply opening 3 so as to flow parallel (in a horizontal direction) to the surface of the silicon wafer W. The provided reactive gas is exhausted to the outside of the apparatus 1 by gas exhaust outlet 13 after passing over the surface of the silicon wafer W to bring about epitaxial film growth.
In recent years, uniform distribution of resistivity within epitaxial film surfaces has become an extremely important quantitative requirement for epitaxial wafers. However, high-temperature processing is required during epitaxial growth. This causes dopant within the wafer to be diffused outwards during the epitaxial growth process and causes a so-called “autodoping” phenomenon where dopant is diffused outwards and is incorporated into the epitaxial film. This causes unevenness in dopant concentration within the formed epitaxial film and causes the resistivity at the outer edge part of the epitaxial film to decrease, and resistivity distribution across the surface to be uneven. In particular, when epitaxial growth is carried out at a concentration lower than the dopant concentration of the silicon wafer W, this causes regions where the dopant concentration of the epitaxial film is outside of the required specifications to occur and causes device yield to decrease.
In order to prevent the deterioration of the resistivity distribution within the epitaxial film, silicon wafers are coated with a protective film so that autodoping from the silicon wafer W is prevented. Silicon oxide films produced by CVD techniques are typically used as the protective films for preventing autodoping and a polycrystalline silicon film formed on the rear surface of the wafer can contribute to gettering capabilities and may also function as a protective film for reducing autodoping. Typically only the rear surface is coated with the silicon oxide film. The edges of the wafer are not coated, but any out diffusion of dopant from the wafer edge is minimal because of the small surface area.
The use of a wafer having a protective film is therefore effective in suppressing autodoping. However, this requires dedicated equipment such as CVD processing tools, etc. and this requires additional processing. There are also cases that demand the use of an epitaxial wafer where the protective layer must be removed from the rear surface after the epitaxial growth process. This requirement depends on the type of processing required. In this case, it is necessary to perform additional processing such as polishing and etching, etc. in order to remove the protective film after the epitaxial growth process. This additional processing causes the cost of producing epitaxial wafers to increase and in recent years this increased cost has made it impossible to produce low cost epitaxial wafers.
An epitaxial wafer that has been processed with an oxide backseal and then the oxide stripped, has a dopant concentration at the rear surface that is similar to the bulk of the substrate. An epitaxial wafer that has been processed without an oxide backseal has a rear surface that is depleted of dopant concentration. This depleted rear surface may be beneficial for subsequent processing by the device manufacturer. In order to resolve these problems, an epitaxial growth process method has been proposed that employs a susceptor 10 formed with a large number of through-holes 10c over substantially the entire surface of the bottom wall of the pocket 10a of the susceptor 10, as shown, for example, in FIG. 23.
However, when there are through-holes 10c dispersed over substantially the whole surface of the bottom wall of the pocket, degradation of the nanotopology of the surface of the epitaxial wafer occurs due to temperature differences between regions where through-holes 10c are formed and regions where through-holes 10c are not formed and these nanotopographical degradation regions occur across the entire wafer surface.
In the prior art, a region from a central position of the bottom wall of this pocket to a radius of ½ is a region for measuring the temperature of the epitaxial growth process in the epitaxial growth apparatus. When through-holes 10c are then formed in this region, variations occur in measurement of the process temperature and as a result there is an increased possibility that slip will occur in the wafer.
On the other hand, uniform epitaxial film thickness is also an important quantitative demand placed upon epitaxial wafers. The aforementioned reactive gas is supplied to the film-forming chamber 2 in a manner parallel with respect to the surface of the silicon wafer W (FIG. 22). Part of the reactive gas flowing into the film-forming chamber 2 therefore collides with the outer wall of the susceptor 10. As a result, the gas flow of reactive gas is disturbed in the vicinity of the upper edge part of the susceptor 10 and it is therefore difficult for the reactive gas to make sufficient contact with the outer edge surface of the silicon wafer W. As a result, this causes a phenomenon to occur where the epitaxial film of this portion becomes thin compared with the surface portion. This phenomenon occurs regardless of whether or not a protective film for preventing autodoping is present at the rear surface of the silicon wafer W.
Methods have therefore been disclosed in the prior art to prevent lowering of film thickness at the outer parts of the epitaxial film through control of the epitaxial growth process. To give concrete examples, there is a method (1) where the speed of growth of an epitaxial film is lowered, and a method (2) where the height D from the surface of the bottom wall of the susceptor 10 to the upper end surface of the sidewall is lowered. This height D is typically 0.55 to 1.00 mm.
However, according to the method (1) of lowering the growth speed, a longer period of time is required to grow the epitaxial film, and this impacts the productivity with which the silicon wafers are produced. Further, when the susceptor height D is lowered in (2), the silicon wafer W being processed may become miscentered in the pocket 10a as the result of small vibrations.
Moreover, the through-holes 10c in the prior art are formed in a direction perpendicular to the bottom wall of the susceptor 10. When the through-holes are formed perpendicular to the susceptor pocket bottom wall, radiant heat can pass through the through-holes and can be absorbed directly on the rear surface of the silicon wafer. This can cause non-uniform heating of the silicon wafer.